Substrate half-reactions

Another important characteristic of ping pong reaction is the presence of exchange reactions in the absence of other substrates. See Isotope Exchange at Equilibrium Multisite Ping Pong Bi Bi Mechanism Half-Reaction Substrate Synergism... 

Deposition of Si02 has been achieved by exposing the substrate to the binary reaction sequence SiCL-l-2H20 —> Si02+ 4HC1. This is divided into the following half-reactions in which species at the surface are inaicated by asterisks [95] ... 

The catalytic activity of MAO can be simply characterized as two half-reactions (Fig. 4.33). In the first half-reaction, the amine substrate is oxidized and the FAD cofactor is reduced. In the second half-reaction, the imine product is released and the FAD cofactor reoxidized generating peroxide. The released imine chemically hydrolyzes to the corresponding aldehyde. 

PING PONG HALF-REACTIONS. Many enzymes operate by double-displacement mechanisms involving covalent enzyme-substrate intermediates as shown in the following scheme ... 

Enzyme-catalyzed reaction schemes involving two or more substrates and/or two or more products in which there are no ping pong half-reaction steps. See Multisubstrate Mechanisms... 

HALF-LIFE HALF-REACTIONS ISOTOPE EXCHANGE SUBSTRATE SYNERGISM PING-PONG MECHANISM COVALENT INTERMEDIATES... 

Both coenzymes undergo reversible reduction of the nicotinamide ring (Fig. 13-15). As a substrate molecule undergoes oxidation (dehydrogenation), giving up two hydrogen atoms, the oxidized form of the nucleotide (NAD+ or NADP+) accepts a hydride ion ( H, the equivalent of a proton and two electrons) and is transformed into the reduced form (NADH or NADPH). The second proton removed from the substrate is released to the aqueous solvent. The half-reaction for each type of nucleotide is therefore... 

At the polymer-electrolyte interface (y = mediation reaction Q + Red <= P-t-Ox into two half-reactions Q+e P and Ox+e Red, where electrons appears explicitly, in order to use the formalism of electrochemical kinetics. For the P/Q couple ... 

The half reactions for the oxidation of formate, sulfate, and xanthine the reduction of nitrate and the reduction of C02 via the carbamate of methanofuran are listed in Table 3b. The potentials for several relevant redox partners are listed in Table 4. On the basis of the respective E° values for the substrate reactions and their physiological redox partners, it is clear that most (if not all) reactions... 

From the sequence of reactions found it follows that copper-quinoprotein amine oxidases catalyze an aminotransferase reaction. A different reaction sequence occurs with flavoprotein amine oxidases (EC 1.4.3.4), where formation of NH3 is not dependent on the presence of 02. However, since reductive trapping of amines in the first half-reaction [86] showed attachment of substrate but not of tritium, the mechanism is also different from the aminotransferase reaction that... 

Benzoylation of benzene and other aromatic compounds by benzoyl chloride over H-BEA zeolite modified by indium oxides has been investigated.191 We report in Table 4.2 the time required for half reaction (L/2) f°r a series of aromatic substrates used in the above reaction. The benzoylation reaction rate (via L /2 value) depends strongly on the substituent group present in the aromatic substrate and increases due to the presence of the electron-donating group, depending upon its electron-donating ability. The activity order is as follows benzene toluene < p-xylene < anisole. 

The substrate half-reactions are displayed in Tables I and II. In each case, a two-electron process seems to be involved. Only in nitro-genase are greater numbers of electrons transferred, and the discussion earlier in this paper summarizes the evidence that these processes occur in two-electron steps. The two-electron reaction of the molybdenum site never appears to be simply an electron transfer reaction. In the case of nitrogenase, each substrate takes up an equal (or greater) number of protons to form the product. In the other molybdenum enzymes, proton transfer and addition or removal of H20 are also required. In each case, however, there is at least one proton transferred in the same direction as the pair of electrons. These data, taken in conjunction with the EPR evidence for proton transfer from the substrate to the active site in xanthine oxidase, suggest that the molybdenum site in all the enzymes... 

Oxo Transfer Mechanisms. Except for nitrogenase, all substrate half-reactions involve the addition or removal of oxygen. The simplest manner of representing these reactions, involves the direct transfer of an oxygen atom to or from substrate, e.g., Reactions 13 and 14. Furthermore,... 

Oxygen is not reduced directly during oxidation of acetate. Rather, pairs of electrons and pairs of protons are removed from the citric acid cycle substrates and transferred to intermediate redox carriers, as shown by the half reactions of Equation 14-2. 

Aminotransferases (transaminases) catalyze the reversible interconversions of pairs of a-amino and a-keto acids or of terminal primary amines and the corresponding aldehydes by a shuttle mechanism in which the enzyme alternates between its PLP form and the corresponding PMP form. In the first half-reaction the PLP form of the enzyme binds the amino acid (or amine) and forms the coenzyme-substrate Schiff s base. Cleavage of the C-a—H bond is then followed by protonation at C-4. Hydrolysis of the resulting ketimine then gives a keto acid (or aldehyde), leaving the enzyme in the PMP form. The latter is recycled to the PLP form by condensation with an a-keto acid, deprotonation at C-4, protonation at C-a and transaldimina-tion to release the a-amino acid formed. 

The kinetics of the half-reactions for pig heart lipoamide dehydrogenase, i.e., the conversion of enzyme to EHj by NADH or dihydrolipo-amide and the reoxidation of EH by NAD or lipoamide derivatives, have been measured by rapid reaction spectrophotometry (24, 137). Reduction of the enzyme by NADH and reoxidation of EH2 by NAD are complete in the dead time of the instrument which is 3 msec. The rate of reduction of the enzyme by dihydrolipoamide is rate determining in the overall reaction and is 33,000 min" at infinite reductant concentration the same rate is determined by conventional kinetics at infinite concentration of both substrates (24) ... 

The vast majority of flavoenzymes catalyze oxidation-reduction reactions in which one substrate becomes oxidized and a second substrate becomes reduced and the isoalloxazine ring of the flavin prosthetic group (Figure 1) serves as a temporary repository for the substrate-derived electrons. The catalytic reaction can be broken conveniently into two steps, a reductive half reaction (from the viewpoint of the flavin) and an oxidative half reaction. The flavin ring has great utility as a redox cofactor since it has the ability to exist as a stable semiquinone radical. Thus, a flavoenzyme can oxidize an organic substrate such as lactate by removal of two electrons and transfer them as a pair to a 2-electron acceptor such as molecular oxygen, or individually to a 1-electron acceptor such as a cytochrome. 

The data thus reveal that ionizable groups are responsible for controlling the rate of flavin reduction. The intermediate and slow kinetic phases of the reductive half-reaction with diethylmethylamine were observed to be essentially independent of substrate concentration above pH 7 below... 

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